The present invention relates to a digital-to-analog converter (hereinafter, referred to as D/A converter).
The present application claims priority from Japanese Patent Application No. 2002-216959, the disclosure of which is incorporated herein by reference.
In recent years, so called “hybridization of technologies” which combines digital techniques with analog ones has been developed and advanced.
For example, in the case of a superheterodyne radio receiver which receives a broadcast from a radio station, at first the receiver converts an intermediate frequency signal (IF signal), which is provided by a frequency converter, into a digital signal. Then, using digital signal processing techniques the receiver carries processing for its signal such as noise-reduction, frequency discrimination, de-emphasis and attenuation. At last, the receiver converts its signal into an analog signal, which is applied to speakers or the like.
Moreover, digital circuits having advanced functions and analog circuits have been able to be formed together on a semiconductor integrated circuit device (LSI). Using such a LSI, the above-mentioned radio receiver can be implemented.
FIG. 1a shows a conventional structure of a semiconductor integrated circuit device 1, which has been proposed for the hybrid superheterodyne radio receiver.
The semiconductor integrated circuit device 1 includes a receiving section 2 into which a received signal Sin from an antenna is input, a de-emphasis circuit 3, a digital attenuator 4, a Δ Σmodulator (Delta-Sigma modulator) 6 and a post-filter 7.
The receiving section 2 includes analog circuits involving an RF amplifier and a frequency converter, an analog-to-digital converter (hereinafter referred to as A/D converter) which converts an intermediate frequency signal generated from the frequency converter into a digital signal, a frequency discriminator for discriminating the frequency of the digital signal with digital signal processing and a matrix circuit or the like.
Then, the digital signal, which is detected and generated with the above frequency discriminator and the matrix circuit, is applied to a de-emphasis circuit 3.
The de-emphasis circuit 3 de-emphasizes the digital signal, because the sounds reproduced from a so called “pre-emphasized signal at a radio station” without a process of de-emphasis are unnatural.
For example, as shown in FIG. 1b, the de-emphasis circuit 3 is formed of a first-order digital low-pass filter which has an attenuation characteristic G1 showing about 6 db/oct in attenuation in the range of frequencies higher than a predetermined cutoff frequency fc1 (specifically about 2-3 kHz). The de-emphasis circuit de-emphasizes the digital signal with the above attenuation characteristic.
The digital attenuator 4 adjusts the level of the de-emphasized digital signal. Then the level-adjusted digital signal is supplied to a D/A converter 5 composed of the Δ Σmodulator 6 and the post-filter 7.
The Δ Σmodulator 6 applies such a modulation as a Delta-Sigma modulation or a Sigma-Delta modulation to the digital signal output from the digital attenuator 4. The Δ Σmodulator 6 outputs code sequences to the post-filter 7, where the high-frequencies of the code sequences are eliminated so that analog voice signals Sout with reduced high-frequency noises are generated.
More precisely, the post-filter 7 is a higher-order low-pass filter implemented using Switched Capacitor Filter (SCF), and has an attenuation characteristic G2 of the low-pass filter in which the attenuation increases sharply with frequency in the frequency range over a predetermined cutoff frequency fc2 (specifically about 20-30 kHz) as shown in FIG. 1c. Due to this low-pass characteristic, the post-filter 7 generates voice signals Sout with the reduced high-frequency noises.
As mentioned above, in the case of the radio receiver also, so called “hybridization of technologies” that combines digital techniques with analog ones has been developed.
The hybridization of digital and analog technologies, as described above, has widely adopted such a type of the D/A converter 5 composed of the Δ Σmodulator 6 and the post-filter 7.
However, there occurs a problem that the circuit scale of the D/A converter 5 becomes large, because the post-filter 7 is constructed by the Switched Capacitor Filters. Further, when the D/A converter is fabricated by semiconductor integrated circuit devices, another high-cost problem occurs because of a large circuit scale of the post-filter 7.
The Switched Capacitor Filter is made of many resistors and capacitors to perform filtering function using charge coupled with specific time constants. Implementation of a D/A converter with semiconductor integrated circuit devices can make use of features of semiconductor fabrication process of being able to make resistors and capacitors with required high-accuracy. Because of this, the Switched Capacitor Filters are employed in a D/A converter.
The Switched Capacitor Filter requires many resistors, capacitors and many switching elements for charge coupling. This causes a problem of a large circuit scale.
Furthermore, D/A conversion of a pre-emphasized digital signal has caused a problem of a larger circuit scale to be required, because of requiring a de-emphasis circuit 3 (digital low-pass filter) at a preceding stage of the Δ Σmodulator 6 and the above mentioned post-filter 7 (Switched Capacitor Filter) at a subsequent stage of the Δ Σmodulator 6.